An understanding of how a brain transforms a mass of sensory signals into a behaviourally appropriate motor response will rest heavily upon understanding the integrative actions of Local Circuit Neurones that are the most numerous central neuronal elements in sophisticated nervous systems. This proposal has two objectives. First, to reveal the design principles of local circuits by analysing the organization of reflex pathways in which local interneurones participate and the types of integrative mechanisms they use. Second, to examine the mechanisms by which the local circuits of different legs are linked by projection interneurones so that a coordinated locomotory movement of all the legs can occur. These objectives can be met by analyzing restricted but repeatable reflexes in a nervous system where these neurones are readily accessible. In an insect, postural and locomotory adjustments of a limb are controlled by two types of individually identifiable local interneurones that have distinct physiological properties and integrative actions. Nonspiking local interneurones control the output of sets of motor neurones by the graded release of transmitter. Spiking local interneurones provide the initial step in processing the sensory inflow from the joints and from the surface of the legs. The local circuits of these interneurones controlling one leg are linked to the local circuits of adjacent legs by a population of identified projection interneurones. The direct contribution of an individual local or projection interneurone to a movement in an alert animal can be assessed. The methods will be physiological to determine by intracellular recording the connections between afferent neurones and local interneurones, between the two types of local interneurone themselves, and between local and projection interneurones. Morphological, by intracellular injection of dyes to reveal the structure of individual interneurones and the regions of neuropile to which they project. Immunocytochemical to label known groups of local interneurones and to identify their putative transmitters. Organizational principles underlying the local circuit control of limb movements should emerge from this broad combination of techniques applied to identified local and projection interneurones.